1. Field of the Invention
This invention relates to a method of forming solder bumps on a substrate, a semiconductor chip, etc. and an apparatus for carrying out the method.
2. Description of the Prior Art
Solder bumps are recently formed by depositing fine solder balls onto a number of metal pads formed on a semiconductor package, for example. The solder bumps are used in a mounting technique for fixing and electrically connecting the semiconductor package. Japanese unexamined patent publication No. 10-64947 (1998) discloses such a method of forming solder bumps as described above. The disclosed method will be described with reference to FIGS. 16A to 16D. A metal plate 2 formed with a number of through holes 1 is placed on a horizontal base 3. A predetermined amount of solder paste is applied to the metal plate 2 using a squeegee 4 so that the through holes 1 are filled with the solder paste. A substrate 6 on which solder bumps are to be formed is attached to the metal plate 2. The substrate 6 and the metal plate 2 are then placed on a heating plate 7 to be heated such that molten solder 8 is transferred to the substrate 6 side.
In the above-described method, however, transfer of the molten solder 8 to the substrate 6 is carried out merely using adhesion to a metal portion of the substrate 6 resulting from surface tension of the molten solder 8. Accordingly, since the molten solder 8 is not positively extruded, the transfer of the molten solder 8 lacks reliability and accordingly, the amount of molten solder 8 transferred is not constant. As a result, the size of the solder bump is not uniform.